This invention relates to compositions and methods for the treatment of metallic surfaces to improve their corrosion resistance and paint adhesion properties. It particularly relates to improved cleaning and titanating pretreatment compositions for the treatment of surfaces of metals such as iron, zinc, aluminum, and alloys of said metals prior to the application of a phosphate (usually zinc phosphate) coating on the surface of the metal.
The treatment of a metallic surface prior to the application of zinc phosphate coating involves the following sequence of steps:
(1) solvent degreasing (optional step, depending on contamination), PA1 (2) alkaline cleaning, PA1 (3) water rinse, PA1 (4) activating and grain-refining. PA1 a titanium salt-from about 0.01 to about 0.18%, preferably from about 0.05 to about 0.15%, calculated as titanium ion; PA1 an alkali metal phosphate-from about 0.6 to about 16%, preferably from about 3.2 to about 12.7%, calculated as PO.sub.4 ; PA1 an alkali metal polyphosphate-from about 3 to about 14.5%, preferably from about 4.0 to about 11.6%, calculated as P.sub.2 O.sub.5 ; PA1 an alkali metal carbonate-from about 1.6 to about 37%, preferably from about 3.0 to about 25%, calculated as CO.sub.3 ; PA1 an alkali metal hydroxide-from about 1.5 to about 11%, preferably from about 3.2 to about 6.4%, calculated as hydroxyl; PA1 an alkali metal salt of EDTA-from about 0.3 to about 9.3%, preferably from about 1.2 to about 6.2%, calculated as EDTA; PA1 an alkali metal silicate-from about 3.5 to about 41%, preferably from about 6.2 to about 37.4%, calculated as SiO.sub.3 ; PA1 the low foaming anionic surfactant sodium butoxy ethoxy acetate-from about 1 to about 15%, preferably from about 2 to about 10%, and PA1 a non-ionic surfactant-from about 0.1 to about 7.5%, preferably from about 1 to about 5%.
Step (4), the activating and grain-refining step is an important step in preparing the surface. Activation involves the conditioning of the metallic surface so that a zinc phosphate coating will form on it readily and quickly. Grain refining involves the conditioning of the surface to be treated so that the crystalline zinc phosphate coating when formed thereon is made up of very fine closely packed crystals. Such surfaces are superior, both in corrosion resistant and paint adherent properties, to coarse grained surfaces. Hereinafter, the term "conditioning" will be used to refer to activating and grain refining.
The most widely used method for conditioning the metallic surface is the application to said surface of an aqueous preparation of certain colloidal titanium salts. These salts, termed "Jernstedt salts", are disclosed in U.S. Pat. Nos. 2,310,239; 2,322,349; 2,456,947; 2,462,196; 2,490,062; 2,516,008 and 2,874,081. An explanation of the action of Jernstedt salts is given in U.S. Pat. No. 3,741,747.
From an operational point of view it was considered desirable to combine the cleaning step (usually carried out at high pH's) and the conditioning step in a single step. However, the stability of the colloidal dispersion of the titanium salts ia adversely affected at high pH's as the colloid tends to break up, with the result that it no longer properly conditions the metallic surface.
The problem with respect to the stability of the colloid at higher pH's has been solved by the addition of stabilizing agents to the cleaning-conditioning composition. Such agents include sodium tripolyphosphate and silicates, preferably in combination, and as described in U.S. Pat. No. 3,741,747, it is possible to obtain cleaning-conditioning compositions which are stable and effective when used in aqueous media at pH's above 10. U.S. Pat. No. 3,741,747 also discloses the inclusion of nonionic surfactants as aids in cleaning and foam control.
The composition is added to water prior to carrying out the cleaning-conditioning step. This water is untreated and invariably contains certain metallic ions, particularly those of the alkaline earth metals, which also adversely affect the stability of the colloidal dispersion of the titanium salts. It has been found, as described in U.S. Pat. No. 3,864,139, that the addition of chelating agents such as, for example, salts of ethylenediaminetetraacetic acid (EDTA) and citric acid, preferably the sodium salts, helps to stabilize the colloidal dispersion from the adverse effects of the alkaline earth and other metals present in the water.
The compositions described in U.S. Pat. Nos. 3,741,747 and 3,864,139 are suitable for the one-step cleaning and conditioning of the metallic surfaces. This step must, to provide effective cleaning and conditioning, be carried out at temperatures in the range of 120.degree.-160.degree. F. The higher end of this temperature range requires a large consumption of energy.